Acoustic admittance testing apparatus

ABSTRACT

Various characteristics of a patient&#39;&#39;s ear at the drum are tested by measuring, at the entrance of the ear canal, the complex acoustic admittance of the ear canal and the admittance at the eardrum together. An acoustic source including a first electro-acoustic transducer provides both an oscillatory air flow and a corresponding signal representing the flow. A second electro-acoustic transducer provides a pressure signal. The inphase and quadrature components of the flow signal are then separately measured relative to the pressure signal while the static pressure in the ear canal is varied so as to controllably stiffen the eardrum, reducing its effect on the total complex admittance.

United States Patent 1 Arguimbau et al.

ACOUSTIC ADMITTANCE TESTING APPARATUS Inventors: Lawrence B. Arguimbau,Lancaster;

Rufus L. Grason, Lincoln, both of Mass.

Grason-Stadler Company, Inc., West Concord, Mass.

Filed: Nov. 1, 1971 Appl. No.: 194,268

Assignee:

U.S. Cl. 128/2 Z, 73/67.l, 179/1 N Int. Cl A611) 10/00 Field of Search128/2 R, 2 Z, 2 S,

128/2 K, 2 V, 2.08; 73/67.1, 69; 179/1 N References Cited UNITED STATESPATENTS 11/1968 Frederik l28/2.08 X 1/1967 Dippolito 128/2 Z 2/1947Summerville et al. 128/2 Z 8/1971 Kahn et a]. 128/2.08

Mendelson 179/1 N X Prall et al. 73/67.]

ABSTRACT Various characteristics of a patients ear at the drum aretested by measuring, at the entrance of the ear canal, the complexacoustic admittance of the ear canal and the admittance at the eardrumtogether. An acoustic source including a first electro-acoustictransducer provides both an oscillatory air flow and a correspondingsignal representing the flow. A second electroacoustic transducerprovides a pressure signal. The inphase and quadrature components of theflow signal are then separately measured relative to the pressure signalwhile the static pressure in the ear canal is varied so as tocontrollably stiffen the eardrum, reducing its effect on the totalcomplex admittance.

21 Claims, 3 Drawing Figures 37 -S-- i PRESSURE H TRANSDUCER 1LOUDSPEAKER L f3l RESERVOIR AIR PUMP

l MICROPHONE 711M, $53

ve a v 1 5| 03c. ATTEN,

5 PHASE 43 2 ADJUST MULTIPLIER, 57

7 PHASE I K59 SHIFTE 6| MULTIPLIER ,63 @2 FILTER I FILTER 2 69 "1"PMENIEUSEH um FROM MICROPHONE T0 45 LOUDSPEAKER 2| ADJUST AVC FILTERACOUSTIC ADMITTANCE TESTING APPARATUS BACKGROUND OF THE INVENTION Thisinvention relates to apparatus for testing at a patients eardrum andmore particularly to such apparatus which functions by measuring thecomplex acoustic admittance at the entrance of the patients ear canalunder different static pressures.

For diagnostic purposes, it is useful for a physician or audiologist tobe able to measure the various acoustic characteristics of a patientseardrum in that various middle ear malfunctions produce recognizablechanges in the acoustic behavior at the eardrum. For example, if theeardrum acts unusually stiffly, this may be an in dication that thestapes, ie the bones transmitting sound from the drum to the fluid inthe inner ear, is held rigid by a bony growth (an indication that thepatient suffers from a condition known as otosclerosis). Similarly, ifthe eardrum shows abnormally high mobility, this is an indication thatsome element in the acoustic chain has become disconnected, ie anossicular discontinuity.

Since the dimensions of the ear canal do not permit ready access to theeardrum itself for direct measurement, acoustic impedance or admittancemeasurements are usually conducted at the entrance to the ear canal andsome compensation must be made for the properties of the canal itself.In the Zwislocki acoustic bridge apparatus, a compensating volume isplaced in series with a comparison standard so that the effect of thecanal is roughly balanced out and the comparison standard can bebalanced against impedance at the patients eardrum. The compensatingvolume to be used is determined in advance by measuring the volume ofliquid required to fill the patients ear canal.

The Terkildsen bridge measures the impedance of the ear canal and drumat normal pressure and then remeasures the impedance while the eardrumis stiffened by the application of a. positive or negative static airpressure. However, only the absolute values of acoustic impedance aremeasured and no allowance is made for the actual complex nature of theeardrums acoustic behavior.

As is recognized in the practice of the present invention, the behaviorat the eardrum is, in fact, best represented by a complex quantity sincethe eardrum/middle ear combination is in fact neither massless norIossless, but rather responds in a damped or lossy fashion. In otherwords, the present invention takes into account the fact that stiffeningthe eardrum by the application of a static pressure may introduce asubstantial phase shift in the response of the totalcanal/drum/middle-ear complex. Accordingly, as measured by the apparatusof the present invention, the change in response due to the stiffeningof the eardrum may be substantially different than that which would berepresented by mere numerical measurement of the absolute values ofimpedances, stiffened and normal.

Among the several objects of the present invention may be noted theprovision of apparatus for measuring the complex acousticcharacteristics at a patients eardrum; the provision of such apparatusin which both inphase and quadrature components of eardrum behavior areaccounted. for; the provision of apparatus for measuring the complexacoustic admittance at the entrance of a patients ear canal for normaland stiffened conditions of the patients eardrum; the provision of suchapparatus which is highly accurate, which is reliable and which is ofrelatively simple and inexpensive construction. Other objects andfeatures will be in part apparent and in part pointed out hereinafter.

SUMMARY OF THE INVENTION In the practice of the present invention, it ispreferably the complex acoustic admittance of the canalldrum/middle-earcomplex which is measured, rather than the impedance. This choicefacilitates the separation of the real and imaginary components of theacoustic response at the eardrum from the response of the ear canal.

Briefly, testing apparatus according to the present invention employs afirst electro'acoustic transducer, coupled to the ear canal, which isenergized to produce an oscillatory air flow in the ear canal, a signalbeing simultaneously provided which is correlated with the flow. Asecond electro-acoustic transducer, also coupled to the ear canal,provides a response signal which is correlated with the acousticpressure in the canal. Means are provided for controllably varying thestatic air pressure in the canal for the purpose of selectivelystiffening the eardrum. Further means are provided for measuring, withrespect to one of the signals, the relative amplitude of the in-phasecomponent of the other signal and the relative amplitude of thequadrature component, at different levels of static air pressure.Accordingly, changes in these components due to stiffening of thepatients eardrum by the varying static pressure can be perceived anddistinguished from components of the response signal which are due toear canal characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram ofa first embodiment of acoustic admittance testing apparatus of thepresent invention;

FIG. 2 is a schematic block diagram of a second embodiment; and

FIG. 3 is a schematic circuit diagram of a modification for theembodiment of FIG. 1 providing increased sensitivity to abrupt changesin signal level.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis indicated very diagrammatically at 11 a patients ear, having an earcanal 13 which communicates with the eardrum 15. An earplug 17 isprovided which is preferably constructed of a relatively compliantmaterial to facilitate sealing. A miniature loudspeaker 21 and apressure microphone 23 are copuled to the ear canal 13 throughrespective tubes 25 and 27 which pass through the earplug 17 and aresealed therein. Both loudspeaker 21 and microphone 23 may be of thehearing aid transducer type.

Plug 17 also accommodates a capillary tube 29 which is connected to aplenum or reservoir 31 which acts as a static pressure source. Reservoir31 is supplied with air at a predetermined rate from a pump 33 operatingthrough a capillary tube 35 so as to generate within reservoir 31 arelatively steady air pressure which rises in controllable orpredictable fashion. A pressure transducer 37 is connected to reservoir31 to provide an 3 electrical signal representing the pressure existingat any given moment.

Loudspeaker 21 is energized from an audio oscillator 41 through acontrollable attenuator 43 so as to provide an adjustable acoustic oroscillatory air flow at the entrance to the patients ear canal. Theelectrical signal provided by oscillator 41 is employed, in theapparatus described hereinafter, as representing the oscillatory airflow in the patients ear canal. As will be understood by those skilledin the art, this signal will be in phase correlation with the air flowin the ear canal, though perhaps not in perfect phase synchronizationtherewith, due to phase shifts and reactive effects in the transducer 21and the tube 25.

A signal representing the pressure present at the entrance of thepatients ear canal is provided by means of the microphone 23, suitablepreamplification being provided by an amplifier 45. As with the flowsignal, the response signal obtained by means of the microphone 23 isproportional to and in phase correlation with the instantaneous pressurepresent at the entrance to the ear canal 13, though perhaps not inperfect phase synchronization therewith due to reactive effects in thetransducer 23 and the tube 27. However, since the determination ofacoustic admittance by the apparatus described herein requires only thedetermination of the complex ratio of the air flow to pressure, a singlephase adjustment in either signal channel can compensate for both setsof phase shifts. An adjustable phase adjuster for this purpose isindicated at 51 in the pressure signal channel.

In the preferred embodiment of FIG. 1, the amplitude of the pressuresignal is maintained substantially constant by means of an AVC circuit53. AVC circuit 53 responds to the average amplitude or envelope of thepressure signal and adjusts the attenuator 45 correspondingly so as tomaintain the pressure signal at a substantially predetermined level. Inother words, a servo-loop is established.

A pair of analog multipliers 57 and 59 is provided for separating, withrespect to the pressure signal, the inphase and quadrature components ofthe flow signal. For this purpose, the constant amplitude pressuresignal is applied directly to one of the inputs of multiplier 57 and toone of the inputs of the other multiplier 59 through a 90-degree phaseshifter 61. The flow signal is applied directly to the other inputterminal of both multipliers. The output signals from the multipliers 57and 59 are time-averaged by means of suitable low-pass or d.c. filters,63 and 65 respectively, to provide a pair of signals which areproportional to the amplitudes of the in-phase and quadrature componentsof the flow signal, respectively. Since the amplitude of the pressuresignal is held essentially constant by means of the operation of the AVCcircuit 53, it can be seen that these d.c. signals properly representsusceptance and conductance directly. These signals may be applieddirectly to suitable meters as indicated at 67 and 69 but, preferably,these signals are also recorded for subsequent study and analysis bymeans of a dual input XY recorder 71, the X axis of the recorder beingcontrolled by the signal obtained from pressure transducer 37 so thatboth the susceptance and conductance signals are separately recorded asfunctions of the static air pressure in the ear canal.

As will be understood by those skilled in the electroacoustic arts, alossless cavity will represent pure susccptancc while factors producingloss or energy absorption will introduce a conductance component.Accordingly, initial adjustment ofthe compensating phase shifter 51 maybe accomplished by coupling the plug 17 to an essentially loss-freecavity, approximating an ear canal, and adjusting the compensation untilno component of conductance is registered.

As indicated previously, the application of a static air pressure insidethe ear canal can effectively stiffen the eardrum so that its compliancedoes not as significantly contribute to the overall admittance of thepatients ear. Accordingly, the contribution to admittance provided bythe eardrum can be recognized through the variations in the complexadmittance produced by static pressure change. However, since bothsusceptance and conductance are measured, it can be seen that thediagnostician has available information which will allow him to separatecomponents of admittance due to compliance (producing susceptance) andviscous loss (producing conductance). Likewise, since the admittance ismeasured as a complex quantity, it can be seen that the full amplitudeof a change in admittance can be recognized whereas the significance ofsuch a change could at least partially be concealed if only absolute ornumerical quantities were measured rather than vector quantities.

In the embodiment of FIG. 2, the in-phase and quadrature components ofthe flow signal are measured by means of a nulling procedure, the staticpressure level being manually adjusted to suitable discrete levels byappropriate means (not shown). In this system, the loudspeaker 21 isdriven at constant amplitude and the pressure or response signal isallowed to vary. The phase shifter 51 which provides for an initialphase adjustment is again employed as is the 90-degree phase shifter 61.Respective preselectably proportions of both the shifted and unshiftedpressure signal are obtained by means of a pair of potentiometers R5 andR6. These components, of adjustable amplitude, are then bucked againstan inverted or negative version of the flow signal in a mixing networkcomprising resistors R1, R2 and R3 which are commonly connected to amixing junction 84. The inverted flow signal is indicated as beingobtained from oscillator 41 through an inverting amplifier 82 but, asunderstood by those in the art, a mere reversal of connections maysuffice.

In order to allow the in-phase and quadrature components to beindependently recognized and nulled, the junction 84 is connected to oneinput terminal of an analog multiplier 85 which, as in the previousembodiment, is employed to effect a phase correlation. The shifted andunshifted versions of the pressure signal are applied to respective AVCamplifiers 81 and 83 so as to obtain corresponding phase referencesignals of substantially constant amplitude. One or the other of theseconstant amplitude signals is applied as the other input signal tomultiplier 85, through a selector switch 87. Since the signals obtainedfrom the amplifiers 81 and 83 are of essentially constant amplitude, theoutput signal from the multiplier 85 is thus essentially synchronouslyresponsive to either the in-phase or the quadrature component of theother input signal applied thereto, ie the unbalance signal present atthe mixing junction 84. The output signal from the multiplier 85 istime-averaged by means of a low-pass or d.c. filter 89 and the resultantd.c. component is displayed on a zero center or null meter 91.

When the switch 87 is in its lower position, the output signal from themultiplier 85 is responsive essentially only to the in-phase componentof the unbalance signal applied to its other input and, thus,potentiometer R5 can be adjusted until a null is obtained. Similarly,when the switch 87 is in its upper position, theoutput signal frommultiplier 85 is responsive essentially only to the quadrature componentof the unbalance signal and the potentiometer R6 may be adjusted toobtain a null. Since the settings of the potentiometers R5 and R6represent the proportions of the response signal required to balance apreset level of reference signal, it can be seen that these settingslikewise represent the susceptance and conductance componentsrespectively of the acoustic admittance of the patients ear. in otherwords, means have again been provided for measuring the complex acousticadmittance of the ear channel plus eardrum, each of the complexcomponents being measured separately. Further, this measurement has beenobtained by means of a multiplier which, through cross-correlation of apair of signals, is made responsive essentially only to either thein-phase or quadrature component at any given moment.

If desired, the acoustic flow rather than the pressure could be heldconstant. The pressure developed by the flow would then be proportionalto impedance rather than admittance. Minor shifts in theinterconnections of microphone preamplifier, drive current, themultipliers and the 90 phase shifter would make is possible to make twometers read directly in total resistance and total reactance. However,for the particular application in hand, the admittance connectionmakespossible an easier correction for the canal.

Apparatus of the present invention is also useful in determiningso-called'acoustic reflex. If a differential amplifier 93 having delayedand non-delayed input circuits as illustrated in FIG. 3 is interposedbetween each meter (67 and 69) of FIG. 1 and its respective input leadit can be seen that the meter reading will be rendered responsive torate-of-change of amplitude as well as to the absolute value ofamplitude. Thus abrupt changes in the signal amplitude, i.e. changesoccurring in a time which is short as compared with the time constant ofresistor R and capacitor C], will be disproportionately amplified. Ascale expansion of :l is appropriate. For relatively slow changes in theamplitude of the input signal, the output signal from amplitier 93closely follows its input signal due to the efiect of the feedbackresistor R13.

As is understood, the acoustic reflex causes the eardrum to be stiffenedby involuntary muscular reflex. Thus if an acoustic reflex is produced,e.g. by means of a stimulus applied to the patients other car, theeffect can be readily perceived in the output signal registered on theappropriate meter. The rate-of-change expansion can also be applied tothe recorded signal as well.

In view of the foregoing, it may be seen that several objects of thepresent invention are achieved and other advantageous results have beenattained.

As various changes could be made in the above constructions withoutdeparting from the scope of the invention, it should be understood thatall matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:

1. Apparatus for testing a patient's car, said apparatus comprising:

a first electro-acoustic transducer adapted to be acoustically coupledto the patients ear canal;

means for energizing said first transducer to produce an oscillatory airflow in an ear canal to which said transducer is coupled and forproviding a signal which is correlated with said flow; means, includinga second electro-acoustic transducer adapted to be coupled to the earcanal, for generating a response signal which is correlated with theacoustic pressure in the ear canal;

means for controllably varying the static air pressure in the canal; andmeans interconnected with both transducers for measuring, with respectto one of said signals, both the relative amplitude of the in-phasecomponent of the other signal and the relative amplitude of the phasequadrature component of said other signal at different levels of staticair pressure in the ear canal, whereby changes in said relativeamplitudes caused by stiffening of a patients eardrum by the varyingstatic pressure are indicative of characteristics of the unstiffenedeardrum. 2. Apparatus as set forth in claim 1 wherein said measuringmeans comprises at least one analog signal multiplier.

3. Apparatus as set forth in claim 2 wherein said means for varyingstatic pressure comprises aplenum chamber and means for providing a flowof air relative to said chamber thereby to generate therein a pressurewhich varies as a function of time.

4. Apparatus as set forth in claim 1 including means for phase shiftingsaid pressure signal and performing a cross-correlation and timeaveraging of said flow signal with both said shifted and unshiftedpressure signals.

5. Apparatus as set forth in claim 4 wherein said means for performing across-correlation and time averaging comprises an analog signalmultiplier and a low pass filter.

6. Apparatus as set forth in claim 1 including AVC means controlled bysaid pressure signal for varying the energization of said firsttransducer to maintain the amplitude of said pressure signalsubstantially at a preselected level.

7. Apparatus as set forth in claim 1 including an adjustable phaseshifter in the path of one of said signals to compensate for phaseshifts which are not a function of the acoustic admittance of thepatients ear.

8. Apparatusas set forth in claim 1 wherein said measuring meansincludes means for differentially amplifying relatively rapid changes inat least one of said components.

9. Apparatus for testing a patients eardrum, said apparatus comprising:

a first electro-acoustic transducer adapted to be acoustically coupledto the patients ear canal;

means for energizing said first transducer to produce an oscillatory airflow of adjustable amplitude in an ear canal to which said transducer iscoupled and for providing a signal which is correlated with said flow;

means, including a second electro-acoustic transducer adapted to becoupled to the ear canal, for generating a signal which is correlatedwith the acoustic pressure in the ear, canal;

means operating in response to the amplitude of said pressure signal foradjusting the amplitude of said oscillatory air flow to maintain saidpressure signal at a substantially constant level;

means for controllably varying the static air pressure in the canal;

means for phase shifting said pressure signal by substantially 90; andmeans for cross-correlating said flow signal with both the phase shiftedand unshifted pressure signals thereby to generate respective admittancecomponent signals, whereby stiffening of the patients eardrum by varyingstatic pressure permits the measurement of the characteristics of theear canal alone and the determination of the effect of the unstiffenedeardrum. 10. Apparatus as set forth in claim 9 wherein saidcross-correlating means comprises a respective analog signal multiplierand low pass filter for each said shifted and unshifted pressure signalswhereby the respective admittance component signals are availablesimultaneously.

11. Apparatus as set forth in claim 9 including means for indicating thevalues of said admittance component signals. 7 7

12. Apparatus as set forth in claim 9 including means for recording thevalues of said admittance component signals.

13. Apparatus for testing admittance at the plane of a patients eardrum,said apparatus comprising:

a first eleetro-acoustic transducer adapted to be acoustically coupledto the patients ear canal;

means for energizing said first transducer to produce an oscillatory airflow of adjustable amplitude in an ear canal to which said transducer iscoupled and for providing a signal which is correlated with said flow:means, including a second electro-acoustic transducer adapted to beacoustically coupled to the ear canal, for generating a signal which iscorrelated with the acoustic pressure in said ear canal;

amplitude control means operating in response to the amplitude of saidpressure signal for adjusting the amplitude of said oscillatory air flowto maintain said pressure signal at a substantially constant level;

means for varying the static air pressure in said canal as a function oftime;

means, including an analog signal multiplier and a low pass filter forcross-correlating said flow signal with said pressure signal thereby togenerate a conductance signal;

means for phase shifting said pressure signal by substantially 90;

means, including an analog signal multiplier and a low pass filter forcross-correlating said flow signal with the phase shifted pressuresignal thereby to generate a susceptance signal; and

means for recording said susceptance and conductance signals as afunction of static pressure.

14. Apparatus as set forth in claim 13 including 8 means fordifferentially amplifying relatively rapid changes in said susceptancesignal and said conductance signal.

15. Apparatus for testing admittance of a patients eardrum, saidapparatus comprising:

a first electro-acoustic transducer adapted to be acoustically coupledto the patiets ear canal;

means for energizing said first transducer to produce an osciallatoryair flow in an ear canal to which said transducer is coupled and forproviding a signal which is correlated with said flow; means, includinga second electro-acoustic transducer adapted to be acoustically coupledto the ear canal, for generating a signal which is correlated with theacoustic pressure in the ear canal;

means for phase shifting said pressure signal by substantially means formixing a first signal which is a preselectable proportion of theamplitude of said pressure signal, a second signal which is apreselectable proportion of said shifted pressure signal, and said flowsignal thereby to obtain a sum signal;

means for controllably varying the static air pressure in said canal;and

means for separately indicating the amplitude of the in-phase andquadrature components of said sum signal with respect to said pressurethereby to permit said components to be nulled whereby the proportionsof said pressure signal and phase shifted pressure signal required to bemixed to obtain a null are representative of the conductance andsusceptance respectively of the patients eardrum and canal and wherebystiffening of the patients eardrum by varying static pressure permitsthe measurement of the characteristics of the ear canal alone and thedetermination of the admittance at the plane of the unstiffened eardrum.

16. Apparatus as set forth in claim 15 including respective adjustablepotentiometers for preselecting the proportions of said shifted andunshifted pressure signals.

17. Apparatus as set forth in claim 15 wherein said mixing meansincludes a resistive mixing network having a series resistance couplingeach of the mixed signals to a summing junction.

18. Apparatus as set forth in claim 15 wherein said means for separatelyindicating includes means for cross-correlating said sum signal witheither the phase shifted or unshifted pressure signal.

19. Apparatus as set forth in claim 18 including AVC means for bringingeither the phase shifted or unshifted pressure signal to a substantialpredetermined amplitude prior to cross-correlation.

20. Apparatus as set forth in claim 18 wherein said cross-correlatingmeans incldes an analog signal multiplier and a low pass filter.

21. Apparatus as set forth in claim 20 including a null meter forindicating the value of the cross-correlation product signal.

1. Apparatus for testing a patient''s ear, said apparatus comprising: afirst electro-acoustic transducer adapted to be acoustically coupled tothe patient''s ear canal; means for energizing said first transducer toproduce an oscillatory air flow in an ear canal to which said transduceris coupled and for providing a signal which is correlated with saidflow; means, including a second electro-acoustic transducer adapted tobe coupled to the ear canal, for generating a response signal which iscorrelated with the acoustic pressure in the ear canal; means forcontrollably varying the static air pressure in the canal; and meansinterconnected with both transducers for measuring, with respect to oneof said signals, both the relative amplitude of the in-phase componentof the other signal and the relative amplitude of the phase quadraturecomponent of said other signal at different levels of static airpressure in the ear canal, whereby changes in said relative amplitudescaused by stiffening of a patient''s eardrum by the varying staticpressure are indicative of characteristics of the unstiffened eardrum.2. Apparatus as set forth in claim 1 wherein said measuring meanscomprises at least one analog signal multiplier.
 3. Apparatus as setforth in claim 2 wherein said means for varying static pressurecomprises a plenum chamber and means for providing a flow of airrelative to said chamber thereby to generate therein a pressure whichvaries as a function Of time.
 4. Apparatus as set forth in claim 1including means for phase shifting said pressure signal 90* andperforming a cross-correlation and time averaging of said flow signalwith both said shifted and unshifted pressure signals.
 5. Apparatus asset forth in claim 4 wherein said means for performing across-correlation and time averaging comprises an analog signalmultiplier and a low pass filter.
 6. Apparatus as set forth in claim 1including AVC means controlled by said pressure signal for varying theenergization of said first transducer to maintain the amplitude of saidpressure signal substantially at a preselected level.
 7. Apparatus asset forth in claim 1 including an adjustable phase shifter in the pathof one of said signals to compensate for phase shifts which are not afunction of the acoustic admittance of the patient''s ear.
 8. Apparatusas set forth in claim 1 wherein said measuring means includes means fordifferentially amplifying relatively rapid changes in at least one ofsaid components.
 9. Apparatus for testing a patient''s eardrum, saidapparatus comprising: a first electro-acoustic transducer adapted to beacoustically coupled to the patient''s ear canal; means for energizingsaid first transducer to produce an oscillatory air flow of adjustableamplitude in an ear canal to which said transducer is coupled and forproviding a signal which is correlated with said flow; means, includinga second electro-acoustic transducer adapted to be coupled to the earcanal, for generating a signal which is correlated with the acousticpressure in the ear canal; means operating in response to the amplitudeof said pressure signal for adjusting the amplitude of said oscillatoryair flow to maintain said pressure signal at a substantially constantlevel; means for controllably varying the static air pressure in thecanal; means for phase shifting said pressure signal by substantially90*; and means for cross-correlating said flow signal with both thephase shifted and unshifted pressure signals thereby to generaterespective admittance component signals, whereby stiffening of thepatient''s eardrum by varying static pressure permits the measurement ofthe characteristics of the ear canal alone and the determination of theeffect of the unstiffened eardrum.
 10. Apparatus as set forth in claim 9wherein said cross-correlating means comprises a respective analogsignal multiplier and low pass filter for each said shifted andunshifted pressure signals whereby the respective admittance componentsignals are available simultaneously.
 11. Apparatus as set forth inclaim 9 including means for indicating the values of said admittancecomponent signals.
 12. Apparatus as set forth in claim 9 including meansfor recording the values of said admittance component signals. 13.Apparatus for testing admittance at the plane of a patient''s eardrum,said apparatus comprising: a first electro-acoustic transducer adaptedto be acoustically coupled to the patient''s ear canal; means forenergizing said first transducer to produce an oscillatory air flow ofadjustable amplitude in an ear canal to which said transducer is coupledand for providing a signal which is correlated with said flow: means,including a second electro-acoustic transducer adapted to beacoustically coupled to the ear canal, for generating a signal which iscorrelated with the acoustic pressure in said ear canal; amplitudecontrol means operating in response to the amplitude of said pressuresignal for adjusting the amplitude of said oscillatory air flow tomaintain said pressure signal at a substantially constant level; meansfor varying the static air pressure in said canal as a function of time;means, including an analog signal multiplier and a low pass filter forcross-correlating said flow signal with said pressure signal thereby togenerate a conductance signal; means for phase shifting said pressuresignal by substantially 90*; means, including an analog signalmultiplier and a low pass filter for cross-correlating said flow signalwith the phase shifted pressure signal thereby to generate a susceptancesignal; and means for recording said susceptance and conductance signalsas a function of static pressure.
 14. Apparatus as set forth in claim 13including means for differentially amplifying relatively rapid changesin said susceptance signal and said conductance signal.
 15. Apparatusfor testing admittance of a patient''s eardrum, said apparatuscomprising: a first electro-acoustic transducer adapted to beacoustically coupled to the patiet''s ear canal; means for energizingsaid first transducer to produce an osciallatory air flow in an earcanal to which said transducer is coupled and for providing a signalwhich is correlated with said flow; means, including a secondelectro-acoustic transducer adapted to be acoustically coupled to theear canal, for generating a signal which is correlated with the acousticpressure in the ear canal; means for phase shifting said pressure signalby substantially 90*; means for mixing a first signal which is apreselectable proportion of the amplitude of said pressure signal, asecond signal which is a preselectable proportion of said shiftedpressure signal, and said flow signal thereby to obtain a sum signal;means for controllably varying the static air pressure in said canal;and means for separately indicating the amplitude of the in-phase andquadrature components of said sum signal with respect to said pressurethereby to permit said components to be nulled whereby the proportionsof said pressure signal and phase shifted pressure signal required to bemixed to obtain a null are representative of the conductance andsusceptance respectively of the patient''s eardrum and canal and wherebystiffening of the patient''s eardrum by varying static pressure permitsthe measurement of the characteristics of the ear canal alone and thedetermination of the admittance at the plane of the unstiffened eardrum.16. Apparatus as set forth in claim 15 including respective adjustablepotentiometers for preselecting the proportions of said shifted andunshifted pressure signals.
 17. Apparatus as set forth in claim 15wherein said mixing means includes a resistive mixing network having aseries resistance coupling each of the mixed signals to a summingjunction.
 18. Apparatus as set forth in claim 15 wherein said means forseparately indicating includes means for cross-correlating said sumsignal with either the phase shifted or unshifted pressure signal. 19.Apparatus as set forth in claim 18 including AVC means for bringingeither the phase shifted or unshifted pressure signal to a substantialpredetermined amplitude prior to cross-correlation.
 20. Apparatus as setforth in claim 18 wherein said cross-correlating means incldes an analogsignal multiplier and a low pass filter.
 21. Apparatus as set forth inclaim 20 including a null meter for indicating the value of thecross-correlation product signal.